Displacement feedback control systems (DFCs) and non-feedback proportional (NFPs) have previously been used for axial piston hydrostatic pumps. The Control Block Diagrams A and B shown in FIGS. 16 and 17 the basic differences between the Feedback (DFC) and non-feedback (NFP) control systems.
The control of the pump necessarily involves controlling the position of the swashplate of the pump. With a DFC mechanism, the swashplate position is primarily a function of only an input signal, manual, electrical or hydraulic. The DFC controls position of a servo piston and swashplate system proportional to the input signal utilizing a mechanical feedback linkage. With an NFP control, the swashplate position is a function of the input signal and the moments imposed on the swashplate which are dependent on the input speed to the pump, the operating pressure for the pump, and the swashplate angle.
The DFC system has several functional advantages, but is expensive to manufacture. The NFP is less expensive but also has certain beneficial characteristics including a softer ride for the vehicle driven by the transmission, and inherent characteristics similar to a low performance anti-stall system. However, on certain types of vehicles, these characteristics can be drawbacks. This is especially true of vehicles requiring constant speed (i.e., constant swashplate position), aggressive performance, responsiveness independent of vehicle load, and applications which use a micro-processor based anti-stall system sensing engine speed. In addition, the NFP control is typically less stable than a DFC due to the lack of compensation provided by the feedback mechanism.
Therefore, it is a principal object of this invention to provide a NFP system to allow improved control for a hydrostatic pump.
More specifically, it is an object of this invention to provide a NFP system to achieve desired operating characteristics which will allow the control of a hydrostatic transmission to closely approximate the control performance provided of a DFC system.
These and other objects will be apparent to those skilled in the art.
A method is provided for allowing a hydraulic pump with a non-feedback proportional control to closely approximate the performance of a displacement feedback control by taking a hydraulic pump including a rotatable piston group engaging a pivotal swashplate, with the pump having associated therewith an input power shaft and a servo piston mechanically connected to the swashplate to influence a torque imposed on the swashplate when rotational power is imposed on the pump, and a neutral return spring connected to the swashplate and a pump housing; providing a valve plate to control fluid flow between pistons in the group and pressure and return fluid conduits; providing in the valve plate a plurality of elongated arcuate slots extending therethrough concentrically located at a constant radius with respect to a center of the plate and an axis of rotation of the piston group, with each slot having opposite ends, an elongated notch at one end of some of the slots and extending away from the end to form a bottom with sidewalls extending upwardly with respect to the bottom; providing a valve plate index for the valve plate to approximately a xe2x88x921.5xc2x0 to xe2x88x920.5xc2x0 wherein the valve plate index is defined as the location of a pressure transition zone relative to top or bottom dead center positions of a piston in the rotatable piston group being in either a fully retracted or fully extended position in its operational movement; providing a cylinder block with a piston port for the piston group with fluid inlet and outlet ports in communication with the pistons of the piston group; providing valve plate crossport of approximately 3xc2x0 to 9xc2x0 wherein the valve plate crossport is defined as the amount of angle of rotation during which the piston port in the cylinder block is connected to both the inlet and outlet ports at the same time; maintaining the swashplate at a first swashplate offset of xe2x88x920.015 in. to +0.015 in. in a first direction parallel to axes of rotation of the piston group; maintaining the swashplate at a second swashplate offset of xe2x88x920.060 in. to +0.060 in. in a second direction perpendicular to a longitudinal axis of the pistons; maintaining the ratio of the volumes of the fluid in the piston bores at a top dead center position of movement of the piston in the bore to the volume at a bottom dead center position of 0.53 to 0.73 and adjusting the spring rate of the return spring to a range of approximately 470-670 pounds/inch; whereby the dependency of the angular position of the swashplate is influenced by less than 50% on operating conditions of the transmission and is increasingly influenced by more than 50% by an input signal acting thereon to ensure stability throughout the transmission""s operating range for speed, pressure and swashplate angle.